Make libc++ the default STL.

Previously, code defaulted to using libstdc++ (which was GNU libstdc++
on the host, or Bionic's "libstdc++" on the target). Now we default to
libc++. The exception for this is when using mingw, because we cannot
build libc++ for mingw.

Change-Id: I68726fa26cdcb6263f17c838d3e859debd57fae1